An engine transmission device comprises a set of moving components ensuring, or involved in, the transmission of the translation of the combustion piston in a cylinder to a rotation of a crankshaft.
Engines are known from the prior art that comprise a transmission device likely to move transversely, i.e., in a direction perpendicular to the translation axis of the combustion piston, in an engine block. This movement originates in the operating clearances existing between the moving components of the transmission device. Such operating clearances are particularly affected by the moving components manufacturing and assembling tolerances, the wear and the deformation under load thereof, and by the differential expansion of the engine parts subjected to different temperatures or made of materials having different coefficients of expansion.
The operating clearances should be perfectly controlled. If they are too big, they lead to excessive acoustic emissions of the engine when operating, accelerated deterioration of its components or even the destruction thereof, for example, caused by disengaging moving components. If they are too small, null or negative, they lead to excessive friction between the moving components and thus to a degraded engine performance, the blocking or even the destruction thereof.
The documents US 2010/206270, EP 1740810 and EP 1979591 disclose devices for adjusting the operating clearance existing between the moving components of a transmission device, with such devices comprising a spring or a hydraulic jack, integral with the engine block, and exerting a transverse force holding the transmission device to keep it in contact with an opposite wall of the engine block.
The above-mentioned documents provide for the application of a static load onto the transmission device. “Static load” means a constant force during an engine cycle. The static load is so calibrated as to oppose the maximum forces that apply to the transmission device, specifically for the engine working conditions (speed, load) generating the greatest forces. The static load ensures the permanent contact between the moving components of the device. It is therefore relatively important.
It should be noted that these documents provide for an embodiment making it possible to control the force exerted, for example, by a hydraulic jack, according to the engine working conditions. In this embodiment, however, when the engine operates under load and at a steady speed, the force exerted by the hydraulic jack is not affected.
Such relatively significant and permanent force induces frictions within the transmission device that affect the engine performance, and impose the adequate dimensioning of the transmission parts, the casing, and the hydraulic power source.
Therefore, it is sometimes chosen to calibrate the static holding force to a level below the maximum forces which apply to the transmission device, but nevertheless at a sufficient level to cover a part of the engine operating range. However, this solution is not satisfactory since it requires using a mechanical stop to limit the operating clearances as soon as the movement becomes excessive.
Upon assembly, such stop requires a fine adjustment specific to each subset associated with an engine cylinder. This operation is particularly undesirable on an industrial scale for reasons of cost.
In addition, the adjusted position of the stop also has the disadvantage of being stationary, and of not compensating for the phenomena related to the differential expansions between the casing and the transmission elements, or the offsets linked to the parts' wear, for example.
When the stop is biased in operation, the shocks are directly transmitted to the motor casing, which induces oversizing, accelerated wear of the impacted parts, and increased noise level.
The need for calibrating the holding force is particularly marked for a variable compression ratio engine, as described in the cited documents of the prior art, according to which a static holding force is applied to one side of a control rack, the longitudinal displacement of which ensures the control of the compression ratio. As a matter of fact, it is particularly important in this case to limit the static value of the holding force so as not to block or limit the movement capacity of the control rack, in particular, by sliding the control rack against the wall of the engine block.